An important practical aspect of modern energy storage devices is ever-increasing energy density and power density. Safety has been found to be a major concern. Lithium ion cells currently in wide-spread commercial use are among the highest energy density batteries in common use and require multiple levels of safety devices, including external fuses and temperature sensors, that shut down a cell in case of overheating before a short circuit can occur as a result of the mechanical failure of the battery separator. Lithium-ion (Li-ion) batteries are also subject to explosion and fire should a short circuit occur because of mechanical or thermal failure of the separator. Li-ion secondary batteries present special challenges concerning durability over many cycles of charge and discharge. Commercially available Li-ion batteries typically employ microporous polypropylene as a battery separator. Microporous polypropylene begins to shrink at 120° C., limiting the battery fabrication process, the use temperature of the battery, and the power available from the battery.
In both fabrication and use, the wound electrochemical devices common in the marketplace impose severe mechanical stressing on the device separator. Those stresses can result in manufacturing defects and device failure. The mechanical stresses can include, for example, strong tension and compression of the separator during manufacture which are used to generate tight winding. After manufacture is complete, the device layers remain under compaction and tensile stress. Further, the manufactured device can be subject to shaking and impact stresses during use.
The requirements for choosing an improved separator for Li-ion batteries and other high energy density electrochemical devices are complex. A suitable separator combines good electrochemical properties, such as high electrochemical stability, charge/discharge/recharge hysteresis, first cycle irreversible capacity loss and the like, with good mechanical aspects such as strength, toughness and thermal stability.
Investigations concerning known high performance polymers for use as battery separators have been undertaken. One such class of polymers has been polyimides.
The Handbook of Batteries, David Lindon and Thomas Reddy, ed., McGraw-Hill, (3rd edition), 2002, describes first cycle discharge capacity loss as an important criterion in secondary batteries (P. 35.19). Also stated is that non-woven separators have been found in general to exhibit inadequate strength for use in Li and Li-ion batteries. (P. 35.29). For this reason, low-melting polyethylene microporous films tend to be used as separators in Li and Li-ion batteries. However, polyethylene microporous films are not thermally suited to the high temperatures occasionally associated with rapid discharge end uses, or end uses in high temperature environments.
Huang et al., Adv. Mat. DOI: 10.1002/adma.200501806, disclose preparation of a mat of polyimide nanofibers by electrospinning a polyamic acid that is then imidized to a polymer represented by the structure.
The mat so prepared is then heated to 430° C. and held for 30 minutes, thereby producing an increase in strength. No mention is made of battery separators.
Kim et al., U.S. Published Patent Application 2005/0067732, discloses a process for preparing polymeric nanowebs by electroblowing of polymer solutions, including polyimide solutions. No mention is made of battery separators.
Honda et al., JP2004-308031A, discloses preparation of polyimide nanowebs by electrospinning polyamic acid solution followed by imidization. Utility as a battery separator is disclosed.
Nishibori et al., JP2005-19026A, discloses the use of a polyimide nanoweb having sulfone functionality in the polymer chain as a separator for a lithium metal battery. The polyimide is described as soluble in organic solvents and the nanoweb is prepared by electrospinning polyimide solutions. No actual battery is exemplified. Heating of the nanoweb to about 200° C. is disclosed.
Jo et al., WO2008/018656 discloses the use of a polyimide nanoweb as battery separator in Li and Li-ion batteries.
EP 2,037,029 discloses the use of a polyimide nanoweb as battery separator in Li and Li-ion batteries.
A need nevertheless remains for Li and Li-ion batteries prepared from materials that combine good electrochemical properties, such as high electrochemical stability, charge/discharge/recharge hysteresis, first cycle irreversible capacity loss and the like, with good mechanical aspects such as strength, toughness and thermal stability.